Conventionally, in applying metallic coating for automobiles or the like, an undercoat such as a primer is painted in advance by electrodeposition, and baked and dried. An intermediate coat is then formed on the baked film. Next, a theremosetting metallic paint (hereinafter simply referred to as metallic paint) is applied on the intermediate coat and a thermosetting clear paint (hereinafter simply referred to as clear paint) is then applied thereon without heating or curing the metallic paint layer, i.e. with wet-on-wet, followed by heating or curing both the metallic paint layer and clear paint layer. The process, after the coating of metallic paint, is specifically called the two-coat, one-bake system.
In order to allow a clear paint to be applied in the wet-on-wet system, a metallic paint must have sufficient surface smoothness and quick drying properties to keep an interface between the clear paint and the metallic paint. Most of metallic paints is a solvent type in which an organic solvent is employed as a diluent. Further, in order to ensure a uniform metallic appearance and a satisfactory finish, a higher dilution ratio is necessary. On the other hand, such a use of solvent-type metallic paint has been increasingly regulated due to the consideration for prevention of contamination, improvements of work environments, and saving of natural resources. Accordingly, a water borne coating composition employing water as a diluent rather than a organic solvent has been recently considered.
It, however, has been impossible to successfully use a water borne coating composition as a metallic paint in the metallic/clear painting system. The reason is that it is difficult to control evaporation of water on spray-coating of the metallic paint. When using a water borne paint, a large scaled equipment is necessary for making the coating atmosphere extremely advantageous for the evaporation of water to result in high cost.
Japanese Patent Publication (unexamined) No. 157358/1971 discloses a method for controlling the viscosity of paint by incorporating crosslinked polymer microgels into a metallic paint. However, this method is not always satisfactory in a wide range of painting conditions. For example, under the conditions where water is used as a diluent, slowly evaporates, that is, under the conditions of a low temperature and high humidity, appearance of the obtained coating becomes poor. One possible cause is that the water content in the metallic paint allows, when clear paint is applied, the clear paint and metallic paint to mix with each other, thus deteriorating the orientation of metallic pigment to result in a poor appearance. Under the conditions where water rapidly evaporates, that is, under the conditions of a high temperature and low humidity, the metallic pigment is roughly oriented because of the rapid increase of viscosity thereby resulting in a poor appearance. Correspondingly, it is not advantageous to omit the preliminary drying or baking from this method after the metallic paint has been applied.